It is estimated that solid catalysts account for about 90% of manufactured chemicals, and thus are extremely important to the chemical industry. It is well known that the performance of a catalyst can be greatly altered by small changes in its properties. Therefore, measuring the rate and products of chemical reactions between gases and catalysts and the characterization of catalysts are important endeavors. The most common and important methods of characterizing a solid catalyst involve the measurement of the adsorption of a gas on and the desorption of a gas from a catalyst.
It is also important to characterize many other types of noncatalytic solids by measuring their interaction with a gas. One such method is to determine the pore structure of a solid by measuring the physical adsorption of a gas near its boiling point, this frequently being done with N.sub.2 (g) near 77 K. For example, the weathering of concrete is influenced by it pore structure. Another example is to determine the strength or quantity of acidic sites on the surface of a solid polymer by measuring the adsorption of a base, such as NH.sub.3 (g).
Reactivity measurements of a catalyst are almost always done at pressures at or above 1 atm, and it is common for such measurements to be done at pressures exceeding 10 atm. In contrast, some of the most important methods of characterizing a solid require measuring the adsorption and desorption of a gas at pressures substantially below 1 atm. One of the most accurate and important of these techniques is termed the volumetric method and requires high vacuum capability (P&lt;5.times.10.sup.-5 torr, 760 torr=1 atm)
The measurement of reactivity typically involves exposing a catalyst to a reaction mixture and measuring the amount and type of products formed. This data yields the activity and product distribution of a catalyst at the given reaction conditions. It is normally desirable to control the rate of flow, temperature, and pressure of reactants in a reactor. By varying these parameters, information can also be obtained on the kinetics of a reaction including the rate constant, activation energy, and orders of the reaction.
Reactors operable at high pressure are made of metal. Since metal is strong, virtually all such reactorrs are rated at &gt;1000 psia (14.7 psia=1 atm). Pressurized containers pose an explosion hazard. They are not combined with good vacuum capability and the volume of the reactor is usually not critical. Virtually all manufactured laboratory scale reactors usuable at high pressure have a volume in the range of 100 mL to 10 L.
A high pressure reaction system will have a high pressure gauge. Since such an apparatus is not designed for measuring the adsorption and desorption of a gas with a solid by the volumetric method, there is no need for also having a highly accurate low pressure gauge. The most accurate high pressure gauges have an accuracy of about 0.1% of full scale, but in the lower end of their range are much less accurate due to noise and drift. Therefore, such a gauge with a range of 1000 psia can measure a pressure to an accuracy of only about 53 torr. Assuming a very small reaction volume of 100 mL and a temperature of 20.degree. C., this corresponds to an error in measuring the amount of gas present of 6.5 mL STP (STP=standard temperature and pressure). Due to the specific experimental steps required when measuring the adsorption and desorption of a gas with a solid by the volumetric method, the repeatability of a gauge is a more meaningful parameter than the accuracy of a gauge for calculating the accuracy of the amount of gas adsorbed on or desorbed from a solid. However, in some cases manufacturers do not give a repeatability specification. A rough value is for the repeatability to be 3-fold better than the accuracy. In this case, the previously described apparatus could be used to measure the amount of adsorption or desorption to an accuracy of about 2.2 mL. This is about 59-fold larger than is acceptable for the least accurate apparatus for measuring the amount of gas adsorbed on or desorbed from a solid utilizing the volumetric method.
The most common and important methods of measuring the interaction of a gas with a solid are to measure the adsorption and desorption of a gas with a solid. Especially important is the measurement of the amount of equilibrium physical or chemical adsorption or desorption of a gas with a solid as a function of pressure. Physical adsorption and desorption data enable the determination of important properties of a solid including its surface area and pore structure. Chemical adsorption and desorption data enable the determination of a variety of properties of a solid including the dispersion and average crystallite size of a supported metal, the number and strength of acidic and basic sites on a solid, the energetics of adsorption and desorption, and the number of catalytically active sites on a solid.
There are two main techniques of doing these measurements: the volumetric method and the flowing gas technique The volumetric method is easily the most accurate, informative, and widely applicable. For both methods there is virtually no limit on the specific (per gram) or total surface area of a solid sample. In particular, samples of specifice surface area from 0.01 to 1500 m.sup.2 /g can be used.
In the volumetric method a dose of gas at an accurately known pressure and temperature is expanded from a dosing volume of accurately known size into a reactor containing a sample at constant temperature. Gas laws enable the calculation of the quantity of gas contained in the dosing volume. Initially He gas is used as the dosing gas, and this enables an accurate determination of the pressure drop caused by the expansion of a nonadsorbing gas. The reactor and dosing volume are then evacuated and the process repeated with an adsorbate. By accurately measuring the gas pressure before and after the valve is opened, it is possible to accurately calculate the amount of gas which is adsorbed on the solid. A permutation of this technique enables the desorption of a gas to be measured.
Since the calculations of gas adsorbed or desorbed involve gas laws, the accuracy and sensitivity of the data are inversely proportional to the sum of the volumes of the reactor plus dosing region plus pressure transducer. For this reason this total volume is kept small, a typical value being 30 mL.
The data are highly sensitive to leaks and outgassing of the apparatus and any residual contamination. Especially if the solid being investigated is a catalyst, the sensitivity to such spurious effects can be great, with the exposure to 0.001 mL STP of contaminant over the lifetime of the measurement (roughly 1 h) sufficient to alter the results in some cases. A scratch or other inadvertent channel to the atmosphere 0.5 cm long by only 2.times.10.sup.-4 cm diameter is sufficient to cause such a leak. Therefore, great attention to details of construction and operation of such an apparatus is required. Standard methods of vacuum technology can result in a system with low leak and outgassing. Residual gases are quickly removed by evacuation. When the measurements are of physical adsorption or desorption, it is common practice to use a mechanical vacuum pump capable of achieving a vacuum of about 10.sup.-3 torr. If chemical adsorption or desorption is being measured, which is generally much smaller in amount and much more sensitive to contamination and leaks, it is common practice to use a high vacuum pump capable of achieving a vacuum of at least 10.sup.-5 torr.
Due to the unreactive surface which they provide, glass vessels are often used for reactions in the low pressure regime. Evacuated glass containers pose an implosion hazard.
The measurement of adsorption or desorption of a gas with a solid by the volumetric technique also requires a very accurate pressure gauge. The accuracy of the results strongly depend on the accuracy of the pressure gauge. The large majority of measurements are made in the pressure range of 0 to 300 torr, but sometimes pressures near 1 atm are used. Therefore, it is common practice to utilize a pressure gauge which reads to about 1000 torr. A mediocre gauge for this type of apparatus and pressure range has an accuracy of about 0.3% of full scale. Gauges of this and much higher accuracy which have full scale ranges of 1 to 1000 torr are readily available. The mediocre gauge can measure a pressure with an accuracy of 3 torr. Assuming a typical reaction volume of 30 mL for an apparatus designed for such measurements and a temperature of 20.degree. C., this corresponds to an error in measuring the amount of gas present of 0.11 mL STP. As noted previously, this results in an error for measuring the amount of adsorption or desorption of about 0.04 mL STP. Apparatus which are of substantially lower accuracy are not operable to meaningfully measure the adsorption or desorption of a gas with a solid by the volumetric technique.
Numerous devices are available to control the reaction of gases with solids at pressures .gtoreq.1 atm. One prior art device is the model CDS 900 bench scale reactor system made by Chemical Data Systems, a division of Autoclave Engineers, Inc. of Oxford, Pennsylvania. This apparatus is capable of monitoring catalytic reactions at pressures between 1 atm and 1500 psia. A sample can be heated at temperatures up to 540.degree. C. Heating and cooling of the reactor are relatively slow. Parr Instrument Company of Moline, Illinois, manufactures a model 4570 stirred reactor which will operate from 1 atm to 5000 psia. A sample can be heated at temperatures up to 500.degree. C. Heating and cooling of this reactor are also relatively slow. Parr and Autoclave Engineers manufacture a number of other devices which operate in the high pressure regime, but none which operate in the low pressure regime.
It is sometimes desirable to evacuate a high pressure reactor. This is done with a mechanical pump. A mechanical pump can reduce the pressure to about 0.01 torr in a reasonable amount of time which results in the removal of 99.999% of the gas in the reactor. Since such apparatus are not designed for measuring the adsorption and desorption of a gas with a solid by the volumetric method, there is no need for also having a high vacuum pump and all of its required gauges, flow paths, and peripheral valves. Whereas high pressure reaction equipment is necessarily robust, high vacuum equipment is relatively easily damaged. Furthermore, as will be described in a following section, due to the fact that the tubing in the high pressure system is of small diameter but large diameter tubing is required for useful operation of a high vacuum pump, even attaching such a pump would not result in achieving high vacuum in an acceptable amount of time. Therefore, the aforementioned prior art can not be readily modified to operate at high vacuum.
Reactors used at high pressures are constructed of metal which has a high thermal conductivity. Therefore, they are also not suitable for immersion in liquid N.sub.2 which is the standard temperature for most measurements of the physical adsorption or desorption of a gas with a solid.
Consequently, there is no prior art apparatus which is capable of operating in the high pressure regime in a manner suitable for running chemical reactions and which can also operate in the low pressure regime in a manner suitable for accurately measuring the adsorption or desorption of a gas with a solid by the volumetric method.
Numerous devices are available to measure the adsorption and desorption of gases with solids at low and ambient pressure regimes by the volumetric method. A prior art device is the model Chemisorb 2800 device made by Micromeritics of Norcross, Ga. This apparatus is capable of monitoring the chemical adsorption of a gas on a solid catalyst in the pressure range of about 1000 to 10.sup.-3 torr. Pretreatment of a sample can be done at pressures down to 10.sup.-5 torr and temperatures up to 750.degree. C. Thorough evacuation of a solid sample is a standard procedure when pretreating a solid prior to measuring the adsorption of a gas on it, and several additional evacuations are also required as part of the adsorption measurement. Evacuation of a solid contained in a sample holder and mounted on a Chemisorb 2800 is relatively slow. In practice, it is found that it takes about 30 to 60 min to reduce the pressure from 1 atm to 1.times.10.sup.-5 torr. Exclusive of the evacuation time, a measurement of the chemisorption of a gas on a solid takes about 1 h. Thus, the time for the evacuations substantially increases the total time of an adsorption measurement. Heating rates of the Chemisorb 2800 are about 10.degree. C./min, and it takes about 1 h to cool a reactor from 650.degree. to 35.degree. C.
Quantachrome of Syosett, N.Y., manufactures a model Autosorb 6 which measures the physical adsorption of N.sub.2 gas at -196.degree. C. on a solid in the pressure range of about 0.1 torr to 1 atm. Pretreatment of a sample can be done at pressures down to 10.sup.-3 torr and temperatures up to 450.degree. C. Micromeritics and Quantachrome manufacture a number of other devices which operate at low or ambient pressure for the purpose of monitoring the adsorption or desorption of a gas with a solid, but none which operate at high pressure.
U.S. Pat. No. 4,489,593 issued in 1984 to Pieters and assigned to Omicron Technology Corporation of Berkeley Heights, N.J., is entitled "METHOD AND APPARATUS FOR DETERMINING THE AMOUNT OF GAS ADSORBED OR DESORBED FROM A SOLID". The apparatus claimed therein is exemplified by the model 100 manufactured by Omicron. This apparatus measures the physical adsorption and desorption of a gas with a solid in the pressure range of about 1000 to 4.times.10.sup.-4 torr by measuring the pressure differential in a dynamic and volumetric manner as gas is continuously added to or withdrawn from the sample chamber. The apparatus can achieve an ultimate vacuum of about 10.sup.-7 torr and a solid sample can be heated to about 450.degree. C. This and other apparatus for measuring the adsorption and desorption of gases which are manufactured by Omicron are inoperable at pressures above 1000 torr.
Catalysts are often affected by exposure to air and require in situ pretreatments which often take as long as obtaining the desired experimental measurement. A sample can not be introduced to any of the aforementioned prior art apparatus without first exposing the sample to air. Therefore, if a sample is pretreated on one apparatus, there is both the inefficiency of transferring the sample to the second apparatus as well as having to repeat the pretreatment on the second apparatus.
There is no prior art apparatus which provides for the accurate measurement of the amount of gas adsorbed on or desorbed from a solid by the volumetric method and which can also function in the high pressure regime in a manner suitable for controlling chemical reactions.
Accordingly, it is an object of this invention to provide an apparatus which can accurately measure the reaction, adsorption, and desorption of a gas with a solid in a pressure range extending from well below to well above 1 atm. It is in particular an object of this invention to provide the apparatus with means to accurately measure the amount of gas adsorbed on or desorbed from a solid by the volumetric method. It is further an object of this invention to provide relatively fast rates of evacuation and a very low ultimate vacuum. In particular, it is an object of this invention to extend the low pressure range to 10.sup.-9 torr. It is an object of this invention that chemical reactions performed at high pressures and adsorption or desorption measurements performed at low pressures can be done on a single sample without removing the sample from the apparatus or exposing it to air.
A second and less accurate and less widely applicable technique of measuring the equilibrium amount of adsorption or desorption of a gas with a solid is defined as the flowing gas technique. This method consists of passing a constant flow of gas over a solid sample at essentially constant temperature while a detector more or less continuously analyzes some parameter of the effluent gas in order to measure the disappearance of a component of the gas flow due to adsorption on the sample or the appearance of a component in the gas flow due to desorption from the sample. More particularly, the analyzer detects the adsorption or desorption of a gas with the solid by more or less continuously monitoring the concentration of the reactant gas which is contained in a large excess of an unreactive carrier gas. The detector is commonly a thermal conductivity detector and the gas flow is almost always at ambient pressure. Some of the analyzers, including a thermal conductivity detector, are capable of operating at high pressures.
For example, a flow of 10% N.sub.2 in He can be flowed over a solid sample at 20.degree. C. Adsorption of N.sub.2 at this temperature will be negligible and thus establishes a baseline for the detector. A cryogenic flask containing liquid N.sub.2 is then put around the reactor. The adsorption of the gaseous N.sub.2 is then monitored at 77K. This information can be used to calculate the surface area of a solid.
Some important properties of solids, such as the pore size distribution, can not be measured by this technique. This technique is also very limited in its ability to measure the amount of adsorption or desorption of a gas with a solid as a function of pressure. The main advantage of this technique is that the requisite apparatus is cheaper than for the volumetric technique, primarily because vacuum capability is not required. A permutation of this technique enables similar measurements of the desorption of a gas.
A generalization of the flowing gas technique involves changing the temperature of the sample at a known and normally constant rate while the analysis is being performed. This method is commonly referred to and is here defined as temperature programmed characterization (TPC). These measurements include temperature programmed desorption, reaction, decomposition, reduction, and oxidation. In virtually all cases the temperature increases during the measurement. The most important of these techniques is temperature programmed desorption, which necessarily requires that the temperature increase with time. This method provides kinetic information on the nature of the desorption process. The reverse method, temperature programmed adsorption, requires that the temperature be reduced in a carefully controlled manner which is relatively difficult and this method is very rarely reported in the scientific literature. This method can provide thermodynamic information on the nature of the adsorption process. None of the aforementioned prior art apparatus can perform temperature programmed adsorption.
The flowing gas technique requires a small system volume. The amount of undesirable band spreading of an adsorption or desorption peak in the gas stream is proportional to the volume of the reactor plus analyzer plus interconnecting tubing. Also, the response time of the technique increases linearly with this volume. For these reasons this volume is kept small, a typical value being about 10 mL.
Control of leaks, outgassing, and contaminants is much harder in this type of system since vacuum technology is not normally used. In contrast to the case for systems which use the volumetric technique, no manufacturer of this type of equipment gives specifications for the degree of leak tightness or contamination of the apparatus. It is a common misconception that a leak from the atmosphere into an apparatus will not occur if the apparatus is at a pressure of ambient or above. In fact, inboard leaks through a sufficiently narrow channel occur at a rate independent of the internal pressure of an apparatus. As previously described, such leaks can seriously effect a measurement.
In order to remove residual gas when the composition of the flowing gas is changed, it is necessary to purge the flow lines for an extended time. The time necessary to purge the lines increases as their volume increases and is also substantially increased by any volume within the lines which is not directly swept by the flowing gas. Virtually all values contain some nonswept volume, and this is especially true of packless valves which are the valves having the lowest leak rate. A time of about 1 h is required to thoroughly purge a system of small volume. This time period, termed the equilibration time, substantially increases the analysis time fsince the additional time to perform TPC is about 1 h.
None of the prior art devices previously described can properly perform TPC. For example, the CDS 900 does not provide the necessary analyzer and flow path for the reactor effluent and the large volume in the apparatus would distort the data and diminish the sensitivity and accuracy of a measurement. The Chemisorb 2800 also does not have a suitable gas analyzer.
An example of an apparatus capable of TPC measurements is the AMI-1 manufactured by Altamira Instruments, Inc. of Pittsburgh, Pa. The AMI-1 uses a thermal conductivity detector. The AMI-1 is designed to flow gas through a glass reactor in the flow range of 5 to 80 mL/min at a maximum temperature of 1100.degree. C. and maximum heating rate of 40.degree. C./min. The apparatus can not do TPC at subambient temperatures. The temperature of a solid sample can not be cooled at a known and constant rate so temperature programmed adsorption can not be performed with this apparatus. No specification is given for the rate of cooling of the furnace. The AMI-1 is only operable near ambient pressure and has neither vacuum nor high pressure capability.
In a recent review of experimental methods and instrumentation for TPC (Alan Jones and Brian D. McNicol, Temperature-Programmed Reduction for Solid Materials Characterization, M. Dekker, Inc., New York, 1986, chapter 3) it is stated that all current apparatus only operate at ambient or subambient pressures, and the use of such a device at high pressure would be of great technical value. Another recent description of some TPC apparatus is provided by Menon (Catalyst Deactivation, Marcel Dekker, Inc. 1988, p. 99). H. Boer et. al. (Rev. Sci. Instrum. 53, 349 (1982)) described one of the very few apparatus which can perform temperature controlled adsorption. None of the apparatus described can also measure the amount of gas adsorbed on or desorbed from a solid sample by the volumetric method.
The AMI-1 also has a gas sampling valve which can be used to pass pulses of a gas over a sample. This enables some measurements to be made of the amount of adsorption of gas by using a thermal conductivity detector to monitor the disappearance of gas from the pulse. However, in most cases the data obtained is less accurate than obtained with the volumetric method and the amount of equilibrium adsorption as a function of pressure can not be measured.
It is therefore a further object of the present invention to provide an apparatus which can perform TPC as well as measure the reaction, adsorption, and desorption of a gas with a solid at low, ambient, and high pressures including the ability to accurately measure the amount of gas adsorbed on or desorbed from a solid utilizing the volumetric method. Another object of the present invention is to greatly lower the equilibration time for TPC. Still another object of the present invention is to provide controlled cooling of a solid sample in such a manner that measurements of temperature programmed adsorption can be performed.
The prior art devices previously described which only operate in the pressure regime at or below about 1 atm have no means of controlling or measuring high pressure gases and the introduction of high pressure gases could easily damage components of the apparatus and possibly cause the apparatus to explode.
Accordingly, it is also an object of this invention to provide an apparatus for measuring the reaction, adsorption, and desorption of a gas with a solid at low and high pressures with protection of the low pressure components of the apparatus from damage when the apparatus contains a gas at high pressure and to provide for safe operation of said apparatus.
Menon (Catalyst Deactivation, Marcel Dekker, Inc. 1988, p. 101) describes the use of a mass spectrometer as the detector for temperature programmed desorption done by the flowing gas technique. A mass spectrometer operates at pressures &lt;10.sup.-5 torr, so it is contained in its own evacuated chamber of large volume. It is necessary to reduce the pressure of the portion of the effluent gas which is analyzed. A common method is to pass some of the gas through a molecular leak so that a very small amount of gas is bled into the evacuated chamber containing the mass spectrometer probe. This pressure reduction leads to a very large degradation of the signal to noise ratio in the mass spectrometer. For this reason some companies, such as UTI of Milpitas, Calif., manufacture an unconventional ion source, called a closed ion source, for the mass spectrometer which reduces but does not eliminate this problem.
Another method of performing temperature programmed desorption is to monitor the gas phase as the sample is evacuated after being exposed to an adsorbate. This method is here defined as temperature programmed desorption by direct evacuation. In particular, in this method a solid sample contained in an evacuable chamber is exposed to an adsorbate. The temperature of the sample is then raised at a known rate while the sample chamber is evacuated and an analyzer contained in an evacuated chamber continuously measures some parameter of the desorbed gas. The standard practice is to place both the solid sample and a mass spectrometer probe within the same large evacuated chamber. This method has much higher sensitivity than the flowing gas technique. However, only samples of relatively low surface area can be used and the apparatus normally only operate at high vacuum. This conventional method of performing temperature programmed desorption differes substantially from the method of the present invention in which the mass spectrometer probe is place in a separate evacuated chamber and the reactor is in air and is isolatable from the evacuated chamber by means of a shutoff valve.
In 1976 Blakely et. al. (J. Vac. Sci. Technol. volume 13, number 5, 109) described an ultra high vacuum apparatus which was modified to enable monitoring chemical surface reactions on single crystals over a wide pressure range. The apparatus incorporates analysis methods used in surface science including low energy electron diffraction, Auger electron spectroscopy, and mass spectrometry which are contained in a large ultra high vacuum chamber. The vacuum chamber was modified to enable a removable cup to be placed over the sample and the cup has gas lines, essentially forming a reactor. The reactor is contained within the large chamber which is at high vacuum. It is alleged that reactions can be done at pressures up to 100 atm within the cup. This arrangement is very cumbersome and is very limited in application. This distinguishes from the present invention, wherein the reactor is exposed to air and is isolatable from vacuum by a shutoff valve.
This device differs in many other ways from the present invention. The apparatus of Blakely only enables reactions to be done on single crystals using samples having a total surface area of about 1.times.10.sup.-4 m.sup.2, corresponding to a specific surface area of &lt;0.01 m.sup.2 /g. However, very few materials are single crystals, no practical catalyst uses single crystals, and all practical catalysts have much higher surface area. A typical value for a practical catalyst is 100 m.sup.2 /g, so that a typical sample size of 1 g has a surface area 10.sup.6 -fold higher than capable of being used in the Blakely apparatus. This apparatus has no sample holder to contain a powdered or pelleted sample, whereas almost all catalysts are of this type. The present invention has no restriction on the specific surface area of a sample and the sample chamber accommodates powdered and pelleted samples. No means is described for cooling a sample in the Blakely apparatus, so neither the surface area nor pore structure of a solid can be determined with this apparatus. The apparatus also has no dosing volume, so measurements can not be made of the amount of adsorption and desorption of a gas with the sample using the volumetric technique. Reactions can only be done in the circulating mode, whereas the present invention enables the more useful methods of the flow mode and batch mode as well as being operable in the circulating mode.
Accordingly, it is also an object of the present invention to provide a means of using a mass spectrometer to monitor the desorption of a gas from a solid of specific surface area from about 0.01 to 1500 m.sup.2 /g in such a manner as to substantially increase the sensitivity of the measurement over the conventional value and achieve this improvement without the complication and expense of a closed ion source.
It is evident that a wide temperature range is encountered in the variety of measurements which are used to measure the reaction, adsorption, or desorption of gases with a solid. For example, measurements of the surface area and pore structure of solids are routinely done at -196.degree. C. Many chemical reactions and pretreatments of solids require high temperatures. For example, gamma alumina, which is an important catalyst support, undergoes substantial changes in its physical properties at temperatures near 1200.degree. C.
With the aforementioned prior art, much time is lost by the relatively slow heating of furnaces to reaction temperature and the subsequent slow cooling of a furnace. The slow thermal response of a furnace is exacerbated if it must be capable of operation at very high temperatures. This is because safety considerations generally limit the temperature which the outer surface of a furnace can have, and this in turn requires additional insulation of a furnace. Larger furnaces in term have a larger heat capacity which slows heating and cooling. For example, ATS of Butler, Pa., manufacturers a model 3110 tube furnace with a temperature rating of 1200.degree. C. This furnace is alleged to heat and cool very fast. The minimum O.D. recommended for the furnace is 8". The heating rate of this furnace is about 40.degree. C./min. Cooling is found to be quite slow below about 100.degree. C. Similar furnaces are available with temperature ratings up to 1650.degree. C. A typical time for pretreating a catalyst or affecting a chemical reaction of a gas with a solid at a temperature of 1200.degree. C. is &lt;1 h. Therefore, the time spent heating and cooling substantially increases the total time of the process.
Therefore, it is an object of this invention to provide an apparatus which can measure the reaction, adsorption, or desorption of a gas with a solid over a very wide range of temperature, including temperatures up to 1650.degree. C., and which can heat and cool a reactor extremely quickly. It is also an object of this invention to provide means for a furnace and insulated container containing a cryogenic fluid to be very rapidly removed and installed on an apparatus so the temperature of a sample can be changed over the range of from -196.degree. to 1650.degree. C. in only a few minutes.
Glass reactors are widely used for chemical reactions. Compared to reactors constructed of metal, glass has the advantages of being transparent, more chemically inert, and capable of withstanding higher temperatures. For example, stainless steel is the most common metal used to construct chemical reactors. The temperature limit for stainless steel for such an application is approximately 500.degree. C., and in some cases deleterious reactions with chemical feedstocks occur at much lower temperature. Reactors made of fused quartz are usable to above 1200.degree. C. Ceramics, such as fused alumina, can be used to construct reactors of substantially higher temperature rating.
The major disadvantages of glass and ceramics are they can not withstand high pressures and are fragile. When a glass reactor is attached to a metal reaction system, there are two main places in which breakage occurs. The first is at the ends of the reactor where fittings couple the reactor to the rest of the apparatus. Breakage occurs due to the glass being crushed by the compressive forces of the fitting. In many cases the reactor has a U shape, with a gaseous or liquid fluid entering one arm and exiting from the other. In this case, a second weak spot is the bottom of the U. Breakage occurs here due to torque transmitted to the reactor when the fittings are tightened. Although various manufacturers claim their fittings to be free of torque, when used with a glass U shaped reactor experience shows that breakage is common.
One method to prevent the first type of breakage is to use a glass to metal seal. Such seals are commercially available, are leak tight, and provide a metal termination for a reactor. In some cases a soft elastomer, such as VITON rubber, is used to make the seal so only modest compressive forces are required. While this reduces breakage of the first type, it does not eliminate it and elastomers are less chemically inert and adsorb and desorb gases more than metal or glass. This lowers the accuracy of measurements of the adsorption or desorption of a gas with a solid made by the volumetric technique.
Breakage of the second type can be reduced by constructing a glass bridge between the two arms of the reactor. However, excessive torque will now cause the reactor to break at the position of the bridge. Micromeritics Instrument Corporation of Norcross, Ga., avoids this problem by using a reactor which has only a single point of attachment to the rest of the system, thereby eliminating the U shape. This is achieved by using concentric tubing to provide means of both entrance and exit for gas. However, this design suffers from the need to have elastomeric O-ring seals within the reactor with the aforementioned undesirable features and in addition the design results in a much lower gas conductance than a design which does not use concentric tubes and this will substantially slow evacuation of the reactor.
It is therefore an object of this invention to provide a glass reactor which is free of elastomers, has a high conductance for evacuation, can be attached to an apparatus for measuring the reaction, adsorption, or desorption of a gas with a solid in a highly leak free manner, and which is highly resistant to breakage.
It is seen that devices for the control and measurement of the flow of gases at high and low pressures are widely used in apparatus which measure the reaction, adsorption, and desorption of gases with solids. Further, control and measurement of the gas environment is crucial to the operation of such apparatus and frequently limits the operating range of such apparatus. In describing the nature of existing devices, it is useful to divide pressure into three regimes: the low pressure regime consists of pressures below 1 atm, the high pressure regime consists of pressures above 30 psia, and the ambient pressure regime refers to the range bounded by 1 atm and 30 psia.
To achieve pressures much below 1 atm requires the use of a vacuum pump. A rotary pump may be used to achieve a pressure of about 10.sup.-3 torr, but lower pressures require both a rotary pump and a second pump of the high vacuum type, such as a diffusion pump or turbomolecular pump. Such pumps can achieve an ultimate vacuum of about 1.times.10.sup.-9 torr, but can not operate above about 0.3 torr. Further, exposure of such pumps to pressures above about 0.3 torr can often damage the pump. For this reason, the attainment of high vacuum requires a rotary vacuum pump, a high vacuum pump, multiple valves, evacuation paths, and a pressure gauge operable near 0.3 torr so that the pressure can be reduced below about 0.3 torr before the high vacuum pump is used to evacuate the chamber.
The measurement of pressure in the low pressure regime is also a specialized field, with various types of gauges being needed to cover the range from 10.sup.-9 torr to 1 atm. None of the gauges useful for measuring very low pressures can operate near 1 atm, and many of them will be damaged by exposure to a pressure above 10.sup.-2 torr. Gauges useful at very low pressures almost invariably operate on an ionization principle and are relatively inaccurate. A separate gauge, usually a themocouple or Pirani gauge, is necessary to measure pressure in the range of about 0.001 to 10 torr.
The design of a vacuum system requires great attention to the materials used with respect to such criteria as their mechanical integrity under vacuum, minimization of the adsorption of gases onto the walls of the system, outgassing of components, diffusion of gases through walls, extreme leak tightness of seals, inner diameter (I.D.) of tubing, and pumping speed. Speed of evacuation is an especially important and difficult design parameter.
At pressures below about 0.1 torr the mass transport of a gas occurs by molecular flow in most containers. The ability of a tube to transport gas is often measured by its conductance. In the regime of molecular flow, the conductance, C, of a tube is independent of the pressure and is approximately given by EQU C=C.sub.m =12.2d.sup.3 /lL/s
where d is the diameter and l is the length of the tube in cm. A tube which is 30 cm long will have a conductance of 87 L/s if its I.D. is 6 cm, and will have a conductance of 7.times.10.sup.-6 L/s if its I.D. is 0.025 cm. At a pressure of 1.times.10.sup.-6 torr, the former conductance corresponds to a rate of mass transport of a gas with a specific gravity of 1 (such as N.sub.2) of 1.times.10.sup.-4 mL STP/s, an extremely low value.
The critical affect of the I.D. of tubing on the performance of vacuum systems can also be demonstrated by the following example. The time to evacuate a volume of size V from an initial pressure of P.sub.i to a final pressure of P.sub.f if the volume is connected to a vacuum pump of pumping speed S by tubing of conductance C is EQU t=(V/S.sub.eff)In(P.sub.i /P.sub.f)
where the effective pumping speed, S.sub.eff, is given by EQU 1/S.sub.eff =1/S+1/C.
A pumping speed of about 100 L/s is typical for a high vacuum pump. Consider a volume of 0.1 L which is to have its pressure reduced from 7.6.times.10.sup.-2 to 7.6.times.10.sup.-4 torr (a factor of 100) and is connected to a vacuum pump of speed 100 L/s by a 30 cm length of tubing with an I.D. of either 6 cm or 0.05 cm. Tubing with an I.D. of 6 cm yields an evacuation time of 0.01 s, and tubing with an I.D. of 0.05 cm yields an evacuation time of 9.times.10.sup.3 s.
In order to achieve reasonable rates of evacuation, it is therefore common for the I.D. of tubing in an apparatus capable of achieving a good vacuum to be at least several centimeters. Tubing used in vacuum systems is generally of wide bore and relatively thin wall thickness.
The proper selection of valves is also critical in an apparatus which functions at low pressure. In order to obtain pressures&lt;10.sup.-7 torr it is generally necessary to use packless valves which are also free of lubricant and are tested to be very leak tight. The orifice of the valves must be of substantial size to avoid restricting the pumping speed of a system. Packless valves are more expensive and available in much less variety than packed valves.
The criteria for the design of apparatus which transport gas at only modest vacuum and at high pressure are quite different. At pressures above about 1 torr the mass transport of a gas often occurs by laminar flow. In this pressure regime, the conductance of a tube depends on pressure and is approximately given by EQU C=C.sub.l =183 d.sup.4 P/l L/s
where d is the diameter and l is the length of the tube in centimeters and P is the average pressure in the tube in torr. A tube which is 30 cm long and has an average P of 760 torr will have a conductance of 6.times.10.sup.6 L/s if its I.D. is 6 cm, and will have a conductance of 2.times.10.sup.-3 L/s if its I.D. is 0.025 cm. A tube which is 30 cm long, has an I.D. of 0.025 cm, and has an inlet pressure of 3 atm and an outlet pressure of 1 atm is found to transport N.sub.2 gas at a rate of 410 mL STP/s. Typical flow rates of gases in laboratory scale gas and reaction systems operating at.gtoreq.1 atm are 0.1 to 30 mL STP/s. Therefore, mass transport is not a problem in this pressure regime.
An additional illustration is provided by the previous example of evacuation time except in this instance the pressure is to be reduced from 760 to 7.6 torr (a factor of 100) and the speed of the vacuum pump is 1 L/s, a value typical of rotary pumps. Tubing with an I.D. of 6 cm yields an evacuation time of 0.5 s, and tubing with an I.D. of 0.05 cm yields an evacuation time of 33 s. Further, the reduction of pressure from&gt;1 atm to 1 atm is rapidly accomplished by venting a chamber and without the use of a vacuum pump.
It is therefore seen that the mass transport of gases in the high pressure regime occurs by a different process and at much higher rates than in the low pressure regime. Further, the ability to transport a given amount of gas is rarely a design concern in the high pressure regime and tubing of relatively small I.D. can be used. Tubing used in pressurized systems is generally of small bore and relatively thick wall thickness.
The maintenance and control of high pressure in a chamber through which gas flows also requires some type of regulating device to isolate the high pressure zone from atmospheric pressure. This is commonly done with a device such as back pressure regulator. It has also been noted that the type of gauge used for measuring high pressure differs from that operable at very low pressure.
Valve selection for pressurized systems is normally straightforward. High pressure valves are usually not rigorously tested for leak tightness under vacuum and possess lubricants. Orifice size is not a concern for the flow rates used in laboratory scale apparatus. For example, a valve designed to attach to tubing of outer diameter 0.25" will pass 1.5.times.10.sup.4 mL STP/s of air with a pressure drop of only 10 psi across the valve. As the pressure rating of an apparatus increases, the selection of valves and components suitable for high vacuum performance rapidly decreases and the price and complexity of the apparatus increases. For these reasons, there are breaks in the design of such an apparatus at pressures of about 200, 1000, and 3500 psia. In particular, at pressures above 3500 psi only packed valves are readily available.
The introduction of more than one pressurized gas into a common volume can cause back flow of one gas into the supply line of another gas. To prevent this hazardous situation, apparatus operating at high pressure contain check valves. This hazardous situation is not of concern in machines operating below 1 atm and a check valve would prevent the evacuation of any volume on the upstream side of the check valve.
It is also common practice to include filters in the flow lines of pressurized apparatus. The filters trap particulate matter which can damage valves and other components. Such particulates can be transported by the flow of gases and liquids in a pressurized machine, but are not readily transported in a vacuum. Conventional sintered disc metal filters, which have a surface area of about 0.04 sq. in. when placed inside of 1/4" O.D. tubing, do not significantly impede the flow of gas in pressured devices. Filters are not normally used in tubing to be evacuated and will normally greatly reduce the speed of evacuation. Filters of large surface area, typically&gt;0.2 sq. in., are also widely available and are used for passing extremely high flows of fluids. Such a filter is contained in a chamber having a volume of about 13 mL. The use of two such filters to protect against particulate matter in a reactor would degrade the accuracy of a measurement by the volumetric technique of the amount of gas adsorbed or desorbed with a solid by a factor of two. It is seen that filters of especially exotic design are required in such apparatus in order not to seriously degrade both the pumping speed and the sensitivity of the apparatus.
It is sometimes necessary to measure the amount of gas adsorbed on or desorbed from a solid sample of significant size, such as a catalyst pellet of 1 cm diameter. This requires a large I.D. of the tubing in the reactor. As previously described, extra volume decreases the accuracy of such measurements utilizing the volumetric method.
It is therefore also an object of this invention to provide a means for lowering the dead volume of a reactor used for such measurements.
It is also sometimes desirable to measure the amount of adsorption at a low equilibrium pressure utilizing the volumetric method. This is easy with manually operated valves, since the operator can throttle the gas flow. However, in a computer controlled apparatus using remotely actuated valves this becomes difficult. A typical time for a remotely controlled packless valve to close is about 0.5 s. If the dosing volume is initially evacuated, then opening a valve to a gas source will cause a gas burst before the valve can be closed. Further, in a well designed apparatus evacuation at pressures about 0.1 torr will be so fast that the pressure also can not be readily set by reducing a higher pressure. An error in setting the pressure of 1 torr at a pressure of 100 torr is inconsequential for adsorption measurements, but is not acceptable if the pressure of the gas in the dosing volume is to be less than about 10 torr.
Accordingly, it is also an object of this invention to provide improved means for setting low pressures in the dosing volume used for measuring the adsorption of a gas on a solid by the volumetric method.
Commercial apparatus for measuring chemical reactions or the adsorption or desorption of a gas with a solid are normally contained in a suitable enclosure. If an apparatus is to be highly multifunctional in its application, then it is likely that occassional changes in the fluid paths will be necessary. In addition, it is desirable to have easy access to the interior components for maintenance and troubleshooting. An enclosure makes such access difficult. A common practice when dealing with apparatus handling gas flows is to have various inlet and outlet valves mounted on the wall of the enclosure. However, this requires that the fittings be disconnected if it is necessary to remove the wall to get access to the interior. Especially if the apparatus is to operate at high vacuum or high pressure, it is most desirable to absolutely minimize the number of fittings which must be manipulated. This presents a challenging design problem.
It is therefore also an object of this invention to provide an apparatus which operates at high and low pressures and which has unusually facile means of being modified and maintained.
It is seen that pressures below 0.001 torr are necessary for important and common methods of characterization of surfaces which involve measuring the adsorption and desorption of a gas with a solid. However, almost all reactions done for the purpose of obtaining a product or evaluating the reactivity of a solid are done at pressures.gtoreq.1 atm. It has been also shown that the physical laws governing the flow of gases, the type of equipment used, and design considerations are very different for the two pressure regimes. It has further been shown that these different and often conflicting criteria are manifest in prior art apparatus.
Consequently, a single apparatus that can accurately measure the reaction, adsorption, and desorption of a gas with a solid at low, ambient, and high pressures is not present in the examples of prior art discussed above.